1. Field of the Invention
This invention relates to an abnormality-detecting device for an internal combustion engine having oxygen sensors provided in the exhaust system for detecting oxygen concentration in exhaust gases at respective locations upstream and downstream of a catalytic converter in the exhaust system.
2. Prior Art
In general, the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is controlled in response to the concentration of oxygen in exhaust gases from the engine detected by an oxygen sensor (hereinafter referred to as the "O2 sensor") which is provided in the exhaust system at a location upstream of a catalytic converter therein, such that the air-fuel ratio of the mixture becomes equal to a desired value.
The O2 sensor is liable to change in characteristics (internal resistance, electromotive force, and response time), due to its deterioration caused by heat and the like. The use of an O2 sensor which is deteriorated in characteristics, adversely affects the accuracy of air-fuel ratio control.
To overcome this inconvenience, various proposals have been made, which include additionally providing an O2 sensor in the exhaust system at a location downstream of a catalytic converter in the exhaust system, in order to compensate for undesirable changes in characteristics of the air-fuel ratio feedback control performed in response to an output from the O2 sensor upstream of the catalytic converter, whereby the air-fuel ratio feedback control is carried out with high accuracy. According to this proposal, in controlling the air-fuel ratio of the mixture supplied to the engine to a desired value in a feedback manner responsive to the output from the upstream O2 sensor, a control amount used in the air-fuel ratio feedback control is increased or decreased for correction based on an output from the downstream O2 sensor, to thereby compensate for deviation of the actual air-fuel ratio control from a value to which the air-fuel ratio of the mixture is to be controlled, due to deterioration of the upstream O2 sensor. However, this proposed solution has a disadvantage that when the upstream O2 sensor has heavily deteriorated beyond the limit of the above-mentioned compensation, it will result in degraded exhaust emission characteristics of the engine.
To overcome this disadvantage, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 4-72438, to detect deterioration of an upstream O2 sensor based on a correction value which corrects the above-mentioned control amount, and a repetition period of inversion of an output from the upstream O2 sensor, to thereby alert the driver for replacement for replacement of the upstream O2 sensor with a new one when it is detected to be deteriorated, so as to prevent the engine from being operated with degraded exhaust emission characteristics.
However, the above proposed method of detecting the deterioration of the O2 sensor has the following disadvantage:
When the catalytic converter is normally functioning, even if the upstream O2 sensor is slightly deteriorated to exhibit slightly degraded performance thereof, exhaust gases are purified by the action of the catalytic converter, and therefore, the exhaust emission characteristics are prevented from being degraded. However, when the catalytic converter is deteriorated in purifying function thereof, noxious components in the exhaust gases are not purified. As a result, even a slight deterioration in the performance of the upstream O2 sensor directly adversely affects the exhaust emission characteristics.
The above proposed method does not contemplate the effect of the deterioration of the catalytic converter upon the exhaust emission characteristics. That is, it merely detects the deterioration of the upstream O2 sensor only by the correction value which corrects the control amount and the repetition period of inversion of the output from the upstream O2 sensor. Consequently, when the catalytic converter is deteriorated in its purifying operation, there is a fear that the engine continues to be operated with degraded exhaust emission characteristics.